Reactivity ratios batch

During copolymerization, one monomer may add to the copolymer more rapidly than the other. Except for the unusual case of equal reactivity ratios, batch reactions carried to completion yield polymers of broad composition distribution. More often than not, this is an undesirable result. 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

Example 13.6 The following data were obtained using low-conversion batch experiments on the bulk (solvent-free), free-radical copol)mierization of styrene (X) and acrylonitrile (Y). Determine the copolymer reactivity ratios for this pol5Tnerization. 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Vinyl acetate-butyl acrylate copolymers (0-100% butyl acrylate) were prepared by both batch and starved semi-continuous polymerization using sodium lauryl sulfate emulsifier, potassium persulfate initiator, and sodium bicarbonate buffer. This copolymer system was selected, not only because of its industrial importance, but also because of its copolymerization reactivity ratios, which predict a critical dependence of copolymer compositional distribution on the technique of polymerization. The butyl acrylate is so much more reactive than the vinyl acetate that batch polymerization of any monomer ratio would be expected to give a butyl acrylate-rich copolymer until the butyl acrylate is exhausted and polyvinyl acetate thereafter. 

Determination of R r and rA batch polymerization of MMA-MAA comonomers afXcarried out for the determination of Rpmax anc reactivity ratios. The same procedure described in the batch process was used for this purpose except that the reaction vessel was a round bottom flask equipped with agitation, a nitrogen gas inlet, an outlet for taking samples, and a condenser. 

Polymerization Results. A batch polymerization of MMA-MAA comonomer was analyzed for the determination of the reactivity ratios of the two monomers. The change in the ratio of the copolymer composition determined by GC was plotted against conversion as shown in Figure 1. Similarly, the calculated curves for some assumed reactivity ratios are also shown in the same Figure. The optimum values of the reactivity ratio for the emulsion poly-... 

Although MAA monomer possesses a larger reactivity ratio than MMA monomer, more MAA was found to exist in the outer side of the particle in the batch latex, as shown in Figures 5 and 6. This behavior could be explained if one can accept the fact that the MAA-rich polymers, which are formed early on during the polymerization, can migrate to the surface of the particle due to their higher hydrophilicity and plasticization of the polymer with the monomer. In the semi-continuous process, it could be expected that copolymer with the same composition as the comonomer feed is formed, and the particle contains a uniform distribution of carboxyl groups. 

Triad sequence assignments have been made for ethyl acrylate-centered triads. Apparent reactivity ratios have been calculated for the semi-batch copolymers using run number theory. A model has been developed to describe the power-feed systems and predict the triad distributions in the incremental and final copolymer using the experimentally determined r-j and r values. 

Samer [104] carried out similar copolymerizations with similar results. An example of his data is given in Fig. 16. Here 2-ethylhexyl acrylate (EHA) was copolymerized with MMA in batch. The miniemulsion polymerizations (two are shown) follow the copolymer equation, while the macroemulsion polymerization gives EHA incorporation that is lower than predicted by the copolymer equation, presumably due to the low concentration of EHA at the locus of polymerization. The dotted hne in Fig. 16 is for a model derived by Samer that accurately predicts the copolymer composition. Samer derived this model by adapting the work of Schuller [149]. Schuller modified the reactivity ratios for the macroemulsion polymerization of water-soluble monomers to take into accoimt that the comonomer concentration at the locus of polymerization is different from the comonomer composition in the reactor due to the water solubilities of the monomers. Samer used the same approach to account for the fact that the comonomer concentration at the locus of polymerization might be different from that of the reactor due to transport limitations of water insoluble comonomers. 

Wu and Schork [152] compared batch and semibatch and mini- and macroemulsion polymerization for three monomer systems, VAc/BA, VAc/dioctyl maleate (DOM) and VAc/n-methylol acrylamide (NMA), with large differences in reactivity ratios and water solubilities. HD was used as the costabilizer. (It should be noted that DOM could function as a costabilizer itself, but for the sake of consistency, HD was added to the DOM polymerizations.) KPS and the... 

However, this does not preclude mini emulsion copolymerization in a CSTR for extremely water-insoluble comonomers. In spite of the fact that the copolymer composition in the continuous miniemulsion is less than that predicted using the homogeneous copolymerization reactivity ratios, the miniemulsion copolymer might be more uniform than the macroemulsion copolymer, where the possibility of significant droplet nucleation could lead to two separate homopolymers or, at the very best, copolymers of various composition. Therefore, it is very important to use CSTR data to scale up a continuous miniemulsion copolymerization product to take into account the different particle growth kinetics for batch and continuous reactors. 

Ek]uation (7-23) calculates the feed composition that yields an invariant copolymer composition as the conversion proceeds in a batch polymerization. Note that comonomer ratios that are near but not equal to the estimated azeotropic value may produce copolymers whose compositions are constant for all practical purposes. The permissible range of feed compositions for which this approximate azeotropy occurs is evidently greater the closer the two reactivity ratios are to each other. 

A thermosetting appliance enamel consists of a terpolymer comprising about 72 parts of vinyl toluene (70/40 meta/para) with about 20 parts of ethyl acrylate (to reduce brittleness of the copolymer) and 8 parts of an acidic vinyl comonomer. The acid is incorporated in the copolymer to provide sites for subsequent cross-linking with a diepoxide. It seems reasonable to expect that grease and slain resistance of the cross-linked enamel will be enhanced if the cross-links are not clustered and almost all initial polymer molecules contain at least one or a few cross-linking sites. To achieve this in a batch copolymerization, what are the best reactivity ratios (approximately) of the major component (vinyl toluene) and the vinyl acid comonomer Show you reasoning. 

The most significant differences between perfectly mixed and segregated flow in a CSTR occur in copoly merizalions. In a batch reaction, the copolymer composition varies with conversion, depending on the reactivity ratios and initial monomer feed composition. In a perfectly mixed CSTR, there will be no composition drifts but the distribution of product compositions will broaden as mixing in the reactor approaches segregated flow. 

This discrepancy is partly due to the fact that random copolymers produced by a batch free-radical polymerization synthetic method can have a significant composition drift if the respective reactivity ratios of the monomers are different.2 This means that the value of the parameter/is not homogeneous in the copolymer layer at the interface. In the PS-PVP case discussed above, the random copolymers directly in contact with the PS or the PVP side of the interface at equilibrium would be PS-rich or PVP-rich, respectively. This segregation of copolymer fractions to their preferred interfaces gives rise to a broadening of these interfaces relative to the case of a random copolymer with a narrow distribution of/values. 

Temperature control policies have also been suggested to diminish the composition spread in batch reactors (92, 94). However, the low sensitivity of the reactivity ratios to temperature (Figure 7), the poor heat transfer characteristics of reacting polymer mixtures (slow response times) and the considerable excursions in temperature (and, therefore, molecular weights) required to maintain adequate uniformity in composition make the application of these temperature control policies unrealistic. 

Experimental System The copolymerisation of styrene with methyl acrylate in toluene using azo-bis-iso- butyronitrile (AIBN) was selected as the model experimental system because the overall rate of reaction is relatively fast, copolymer analysis is relatively simple using a variety of techniques and the appropriate kinetic and physical constants are available in the literature. This monomer combination also has suitable reactivity ratios (i = 0.76 and r4 =0.175 at 80 C),(18) making control action essential for many different values if compositionally homogeneous polymers are to be prepared at higher conversions in a semi-batch reactor. 

The simplest diblock polymer, styrene/butadiene block polymer, is formed when the two monomers are charged into a batch reaction along with the catalysts. The reactivity ratios are such that the butadiene polymerizes first and with almost total exclusion of any styrene present. Only after all of the butadiene monomer has been consumed does the bulk of the styrene enter the polymer chain. 

If reactivity ratios are particularly disparate then it is possible to form a block copolymer from a batch polymerization. Thus the copolymerization of MAH with S by NMP or RAFT with excess S provides P(MAH-o/f-S)-i(> ocA -PS. There is a similar outcome in other copolymerizations which show a strong alternating tendency such as S with maleimides e.g. or AN. The... 

In addition to the above investigations, free-radical high-pressure polymerizations should also be studied in continuously operated devices for three reasons. (1) Because of the wealth of kinetic information contained in the polymer properties, product characterization is mandatory. Sufficient quantities of polymer, produced under well defined conditions of temperature, pressure, and monomer conversion, are best provided by continuous polymerization, preferably in a continuously stirred tank reactor (CSTR). (2) Copolymerization of monomers that have rather dissimilar reactivity ratios, such as in ethene-acry-late systems, will yield chemically inhomogeneous material if the reaction is carried out in a batch-type reactor up to moderate conversion. To obtain larger quantities of copolymer of analytical value, the copolymerization has to be performed in a CSTR. (3) Technical polymerizations are exclusively run as continuous processes. Thus, in order to stay sufficiently close to the application and to investigate aspects of technical polymerizations, such as testing initiators and initiation strategies, fundamental research into these processes should, at least in part, be carried out in continuously operated devices. 

More often than not, reactivity differs from monomer to monomer. This is evident when the reactivity ratios differ from a value of one. Thus, if one is operating at concentrations other than the azeotropic composition, batch copolymerization will result in a changing copolymer composition throughout the reaction. For example, a copolymerization with rj > 1 and r2 < 1 would result in the instantaneous copolymer composition decreasing in monomer 1 as monomer conversion increases. The degree of compositional drift that leads to a heterogeneous copolymer composition depends on the ratio of reactivity ratios where heterogeneity increases with the... 

Tip 13 (related to Tip 12) Copolymerization, copolymer composition, composition drift, azeotropy, semibatch reactor, and copolymer composition control. Most batch copolymerizations exhibit considerable drift in monomer composition because of different reactivities (reactivity ratios) of the two monomers (same ideas apply to ter-polymerizations and multicomponent cases). This leads to copolymers with broad chemical composition distribution. The magnirnde of the composition drift can be appreciated by the vertical distance between two items on the plot of the instantaneous copolymer composition (ICC) or Mayo-Lewis (model) equation item 1, the ICC curve (ICC or mole fraction of Mj incorporated in the copolymer chains, F, vs mole fraction of unreacted Mi,/j) and item 2, the 45° line in the plot of versus/j. 

The emulsion copolymerization of vinyl acetate and butyl acrylate has received considerable attention. The butyl acrylate confers improved film forming characteristics to the polymer. The disparities in their water solubilities and of their individual polymerization rates may help to explain the variations in reactivity ratios that have been reported [170,171]. The variation in reactivity ratios may also by related to the following observations The reaction method has an effect on the morphology of the polymer particles. In a batch emulsion process, a butyl acrylate—rich core is formed which is surrounded by a vinyl acetate-rich shell, in a process in which the monomers are fed into the reactor in a semicontinuous manner, particles form with a more uniform distribution of the monomers [172]. The kinetics for a batch process indicates that the initially formed polymer is indeed high in butyl acrylate. As this monomer is used up, eventually a copolymer high in vinyl acetate develops. It is this latter polymer which forms the final shell around the particles. 

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